The Six Fundamental Parameters of Machine Vision Optics
If you are like most people working with machine vision, you’ve probably wondered how different lenses can help you get better inspection and defect detection capability from your vision system. How much do you need to invest? Can you get the resolution you need from a high-quality, yet inexpensive lens? These are important questions and critical to making smart lens choices.
The first thing you need to do is familiarize yourself with the fundamental parameters that influence image quality. This will go a long way in maximizing the performance of your vision system.
Here are the six principal parameters:
1. Field Of View: The viewable area of the object
Field of view (FOV) can be measured by the relationship between the camera’s sensor and chosen lens. You might be familiar with the fact that lenses have a given focal length in millimeters. The focal length of a lens tells us what the angular field of view is in degrees. Without getting too into the physics, a rule of thumb to consider is that the longer the focal length, the narrower the angular field of view and the higher the amount of magnification. The shorter the focal length, the wider the angular field of view and the lower the amount of magnification.
2. Working Distance: The distance from the front of the lens to the object
The working distance between the front of the lens and the part that is to be inspected is highly dependent on the application. Are you limited in space in your application, forcing the vision system or sensor to be mounted closer to the part or feature? Or can you be more flexible in physical placement of your vision system? These scenarios should be considered when choosing the lens for your application.
3. Resolution: The ability of an imaging system to reproduce an object or feature’s detail
Image sensors are divided into a number of pixels in the horizontal and vertical dimensions of the sensor, each pixel of a certain size in micrometers. You can use these specifications to determine the resolution of the sensor and if it is right for your application. In general, the smaller the object or feature to be inspected, the more resolution is required.
Depth of field can be an important parameter in very specific applications. For example, barcode reading applications in logistics environments require large depths of field. This is true because the objects that are within the FOV can often be of different sizes and scales, placing barcodes at various working distances relative to the focal point of the lens.
A lens characteristic related to this concept is its aperture, or the opening in the lens that allows light to reach the camera’s image sensor. Aperture is commonly referenced as an f number or f stop and different values affect the amount of depth of field for a given focus point. A smaller aperture (larger f number) allows less light into the sensor and results in a larger depth of field. A larger aperture (smaller f number) allows more light into the sensor and results in a shallower depth of field.
Image sensors come in a variety of different sizes anywhere from 1 inch to as small as ¼ inch.
The horizontal dimension of an image sensor can be used, along with your desired FOV in mm, to determine the necessary primary magnification (PMAG) for your application.
As stated in the previous section, imaging lenses have a focal length that determines the lens’s angular field of view and the amount of magnification of the resulting image. This is known as primary magnification, or PMAG. Using the horizontal dimension of the image sensor and the desired FOV, you can determine the necessary PMAG value, which will help in choosing a lens for your application. For example, you can use the following equation: